The present invention relates generally to fluid infusion systems, and more particularly to an improved medical infusion pump including several flow sensing devices.
The infusion of fluids into the human body is usually accomplished by means of an administration set in conjunction with a metering device which controls the rate of flow of fluid through the set. Peristaltic-type pumps, which function by repetitively compressing and expanding a section of tubing, have been frequently used since they do not introduce the possibility of leakage or contamination into the system while providing positive control of fluid flow through the system.
One such pump is described in U.S. Pat. No. 4,217,993 entitled "Flow Metering Apparatus for Fluid Infusion System". The patented pump construction includes individually spring-biased rollers in the pump rotor which provide a compression force to "squeeze" the fluid through the tubes in a pumping action. The metering apparatus disclosed in U.S. Pat. No. 4,217,993 includes a control system which allows the volume and rate of flow of the fluid to be infused to be preset by the operator. This control system provides an alarm function, in the event of malfunction of the pump or occlusion of the tubing of the administration set, and establishes a minimum pumping rate for maintaining fluid flow through the system after the desired volume of fluid has been infused. A bubble detector circuit is also described in U.S. Pat. No. 4,217,993. This detector comprises a multi-vibrator operatively connected to light generating and detecting components which compare the effects of bubbles in the fluid against a threshold value and sound an alarm when the threshold is exceeded.
Thus, the prior art in general teaches the use of a peristaltic infusion pump with associated sensors, but such apparatus known to date are not totally satisfactory.
Conventional peristaltic pumps have a reputation for excessive cell damage to fluid such as blood due to the continual strong compression of the inner diameter of the tubing contacted by the roller or equivalent structure. Also, such pumps often produce inaccuracies in the amount of fluid delivered because of small physical variations in the tubing introduced during manufacture, such as shore hardness variations, tubing symmetry, and variations in the relationship between the inner and outer diameters. Also, the heavy compression over an extended section of the tubing by the roller or equivalent structure often produces spalling, i.e., the flaking off of small particles of tubing after extended use.
Pressure sensors in conventional infusion pumps typically are either very expensive or are not sufficiently accurate. Typical low cost detectors have a measurement range of 4-8 psi plus or minus 4 or more psi.
Air-in-line, or bubble sensors are also known, but conventional sensors have had problems relating to repeatability of signal from administration set to administration set, and are characterized by inconvenience of inserting the tubing into the sensor device.